Pressure relief valve bad?

We have a Valiant oil boiler for our heat, which, being late July in the Philadelphia area, hasn't been running since around April. The other day, we noticed some dampness on the concrete floor around the boiler and where the sewer line exits the house. The water smelled a bit strong, like ammonia or cat pee (we have two cats and we just moved their litter to the basement), so originally I thought we had a leak or the cats decided they didn't like litter anymore.

I think I ruled out the leak in the sewer line because it hasn't rained in about two weeks and when it did rain in June (for about 15 days straight) there wasn't a problem. Coupled with the fact that the water was negligible and my wife did about six loads of laundry and ran the dishwasher twice over two days, I think the pressure relief valve might be bad on the boiler. I didn't notice any dripping and didn't think it could be the boiler until last night, but we stuck a bucket under it and it probably dripped about one to two pints between 11 p.m. and 6 a.m. I checked the boiler pressure, and it was sitting around 5 psi. The boiler is probably about 10 years old, and we had to replace both the aquastat and circulator pump this winter. I'm guessing it's not a stretch that the valve could be bad.

Does that make any sense? I'm getting ready to call the boiler guys to come round, but I'm wondering if I shouldn't get to excited about it being resolved lest I have to prepare for the shock of a new sewer line.

Most boilers run on one atmosphere (around 14-15 pounds), so 5 is low. Is the boiler turned off? Many, not all by any means, have an automatic fill valve that will maintain it at the set pressure. If it is not on and maintaining temp, then if it is leaking from that valve, you probably need to replace the valve. The valve is designed to drain for two conditions: excess pressure and/or excess temperature. If the boiler is off, it certainly isn't excess temperature. So, if you've determined it really is coming from that valve, and your pressure really is 5#, the valve is probably shot.

If the boiler is on to provide domestic hot water (say from an indirect or a tankless), then it sound like you may have a bad expansion tank. WHen it runs, the pressure rises, causing the valve to open. Then, the system cools off in between (some do, not all), the heating water shrinks, relieving the pressure, and now it is low. Go through too many cycles like this without an auto-fill valve, and the pressure will be too low and you'll have problems with the system producing steam when it wasn't designed for it.

If it maintains temp, it's likely 5# would prevent it from operating and you'd get a low-pressure sensor indication. I'd also check the boiler's expansion tank. If you tap on it, it should feel light, like it is full of air. If it is heavy, the bladder may be shot, and that can have issues with pressure. If the boiler is trying to maintain temp, and the pressure is low, it will boil the water, which would deplete the volume if it doesn't have an automatic fill-valve, leading to a low pressure situation, and water being expelled. Steam would generate the pressure to cause it to release if the temp didn't get high enough. Then, when the low pressure after shut-down happened, would prevent it from starting up again.